This invention relates in general to the sealing of containers, and more particularly to a machine and process for heat sealing membranes to containers.
It is most important to have food containers tightly sealed so as to prevent infestation by microorganisms. One of the more common means for obtaining a tight seal is the common screw cap which is run down tightly against the upper rim of the container to which it is applied, thus forming secure seal at that point. However, the screw provides a spiral path along which microorganisms migrate to the periphery of the seal, so that once the cap is removed, the microorganisms have little difficulty migrating the rest of the way into the interior of the container. Furthermore, screw caps are difficult to handle and must be twisted in order to be applied. This requires complicated and expensive machinery. Often, the machinery applies too much torque to the cap, making the caps extremely difficult to remove.
Plastic lids which snap onto the upper rims of their containers are a relatively recent innovation which has gained considerable popularity in connection with butter, margarine and other spreads which are normally sold in plastic tubs. These snap-type lids are relatively easy to apply and the containers themselves do not have spiral paths along which microorganisms can migrate. Furthermore, the snap lids are easily removed.
It is not uncommon to interpose a membrane-type seal between the rim of the container and the top of the lid, whether the latter be a snap lid or a screw lid. This seal accommodates for irregularities in the opposing surfaces of the container rim and screw cap, and usually remains in place after the lid is removed, thus preventing unauthorized sampling from store shelves. Some of these membrane seals are even heat sealed to their containers to insure against infestation and unauthorized sampling. The usual procedure for obtaining a heat seal is to place a foil coated with a heat sensitive adhesive over the rim of the container and then apply pressure to the foil with a heated platen. The heat is transferred to the foil by conduction and activates the adhesive.
Where the container is formed from a flexible plastic and has a relatively large top as is the case with a margerine tub, the seal must be applied while the plastic lid is on the container since without the lid, the axial sealing force applied to the foil to seal it will distort the container into an elliptical configuration which does not match the circular shape of the foil membrane. In other words, the cap confines the container at its upper rim and thereby enables the container to maintain its shape in the presence of the force. However, most plastic lids will not conduct sufficient heat at low temperatures to activate the heat sensitive adhesive on the foil. Of course, when the temperature is elevated high enough to insure good transmission, the heat will destroy the plastic lid.
Heretofore, the seal membranes have been inductively heated by passing the containers with the seal membranes and lids in place through a magnetic field while rotating the containers to insure even heating of the foil membranes. This is a slow and complicated procedure which is not compatible with high speed filling and capping lines.